"The police can set up a road-block and demand that drivers provide a breath test and proof of their license at any time. Isn't that a presumption of guilt rather than innocence?"
Not really, it's just a requirement to be subject to testing. Like if there is a rule saying you have to take your car to the mechanic once a year to test the brakes and indicators, it's not presuming anything about you being guilty.
"The taxman can deliver an assessment that says you owe $xxxxx in taxes and you are presumed to be guilty unless you can prove you don't owe that much in tax."
Well, that's not a criminal issue. There is no presumption bias in civil matters.
"Here in NZ, Kim Dotcom (love him or hate him) has had his assets seized and was incarcerated at the US government's whim"
Last I checked he is not incarcerated, and most of those assets were seized by the US, not NZ, but I take your point.